1. Field of the Invention
The present invention is directed to a radio-frequency shield for a diagnostic magnetic resonance apparatus, the shield being of the type having a hollow-cylindrical carrier composed of a dielectric, having a first, electrically conductive coating arranged at an inside of the carrier and provided with axially aligned separating slots, a second electrically conductive coating at an exterior of the carrier and provided with axially aligned separating slots, with the separating slots in the first coating and in the second coating being offset relative to one another in a circumferential direction, and wherein the first coating has slotted end regions at the ends of the carrying member and at least one slotted middle region between the end regions.
2. Description of the Prior Art
A radio-frequency shield of this type is disclosed in U.S. Pat. No. 5,574,372. The radio frequency shield is arranged between a gradient coil system and a whole body radio-frequency antenna. Its function is to shield the radio-frequency field of the whole body radio-frequency antenna from the exterior and to keep disturbances originating from the exterior out of the imaging region of the magnetic resonance apparatus. A completely closed, conductive envelope would in fact be optimum for the radio-frequency shielding effect, but is unusable because the radio-frequency shield must be transmissive for the low-frequency gradient fields on the order of magnitude of up to 20 kHz. In order for the low-frequency gradient fields to be built up substantially undistorted in the imaging volume, it is necessary to substantially suppress eddy currents induced by the fields in the radio-frequency shield. To that end, the conductive layer acting as radio-frequency shield is slotted in the longitudinal direction, i.e., in a principal direction of the mirror currents generated by the antenna in the radio-frequency shield. The leaks in the radio-frequency shield which arise due to the slots are shielded by a second conductive layer wherein the separating slots are arranged offset in the circumferential direction from the slots in the first coating. The two layers or coatings are applied on opposite sides of a tubular carrier composed of a dielectric material. Radio-frequency currents can flow across the slots due to the capacitive coupling. Since the gradient coils extend beyond the exterior dimensions of the whole body antenna in the axial direction, the longitudinal dimensions of the radio-frequency shield are also basically adapted to those of the gradient coils. The design of the separating slots is essentially dependent on whether a linearly or a circularly polarizing whole body antenna is to be shielded. A disadvantage, however, is that local heating can occur in the radio-frequency shield, particularly given rapid imaging or given high gradient field strengths.
Another possibility for the design of the radio-frequency shield is disclosed in U.S. Pat. No. 5,680,046. C-shaped structures are applied double-sided on a carrier in order to simulate the radio-frequency mirror current paths as exactly as possible. A good radio-frequency shielding with a simultaneously good suppression of the eddy currents thus is achieved. A disadvantage, however, is that the shielding effect is only effective for a specific radio-frequency antenna. Thus, the shield design must be implemented differently when a circularly polarizing antenna is to be shielded instead of a linearly polarizing antenna.